1. Technical Field
The present invention relates to a nitride semiconductor laser device.
2. Description of Related Art
An SCH (Separate Confinement Heterostructure) is a widely used structure in semiconductor laser diodes. In SCH laser diodes, the injected carriers are confined to the active layer by the band step at the interface between the active layer and the optical guiding layer, while the optical field is confined to the optical guiding layers and the active layer by the refractive index step at the interface between the optical guiding layer and the cladding layer. In the laser diodes composed of III-V nitride semiconductor, which is described by the notation of (AlxGa1−x)1−yInyN (0≦x≦1, 0≦y≦1), the SCH is used as well as laser diodes composed of conventional semicondutors. However, achieving good optical field is difficult in the III-V nitride semiconductor laser diodes because of its difficulty of obtaining the high quality AlxGa1−xN epitaxial layer with sufficient thickness and Al mode fraction for the optical confinement. Therefore, the threshold current density and FFP (Far-Field Pattern) of III-V nitride semiconductor laser diodes are inferior to that of the laser diodes composed of conventional semiconductors because of its poor optical confinement.
As shown in FIG. 1, the conventional III-V nitride semiconductor laser diode has a multiple layer structure epitaxially grown on a substrate 1 of single crystalline sapphire. FIG. 1 also shows the multiple layer structure of the III-V nitride semiconductor laser diode of an embodiment of the instant invention which will be described later.
The spitaxial layers consist of a GaN or AlN buffer layer 2 grown at a low temperature, an n-type GaN base layer 3, an n-type ALGaN cladding layer 4, an n-side GaN optical guiding layer 5, an active layer 6 essentially consisting of InGaN, and AlGaN electron barrier layer 7, a p-side GaN optical guiding layer 8, a p-type AlGaN cladding layer 9, and a p-type GaN contact layer 10.
An n-electrode 12a and a p-electrode 12b are deposited on the base layer 3 and the contact layer 10 via windows of an insulating layer 11, respectively. In order to get high-quality and smooth single crystalline layers, the buffer layer 2 is firstly grown in the sapphire substrate 1. The base layer 3 is grown as contact layer for n-electrode because the sapphire substrate 1 is an insulator.
As described above, the light confinement of the conventional SCH III-V nitride semiconductor laser diodes can be improved by (1) increasing the thickness of the cladding layer 4 or (2) lowering the refractive index of the cladding layer 4.
With the scheme (1) in use, in the case where the cladding layer 4 of AlGaN having a smaller lattice constant than GaN is formed on the base layer 3 of GaN, tensile stress is produced inside the cladding layer 4. This makes it easier to form cracks. When the thickness of the cladding layer 4 becomes large, particularly, the tendency becomes prominent. Such cracks in the cladding layer 4 degrades the emission characteristic of the laser diode.
A strain relaxing layer (not shown) grown between the base layer 3 and the cladding layer 4 relax the lattice unmatch. Using the strain relaxing layer, the cracks can be reduced in the cladding layer 4 and the thicker cladding layer 4 can be achieved. For example, the strain relaxing layer made from InGaN and a thickness is about 0.1 to 0.2 μm. However, distortion energy for relaxing lattice mismatch is stored in the strain relaxing layer as a strain and significantly degrades the crystal quality of the strain relaxing layer. In addition, the strain stored in the strain relaxing layer produces new dislocations, and it degrades optical gain in active layer. Therefore, the threshold current density increases.
Further, the scheme (1) is disadvantageous in increasing the grouth time for the cladding layer 4 as well as increasing the thickness thereof, thus increasing the production cost.
According to the scheme (2), the refractive index of the cladding layer 4 can be reduced by increasing the Al mole fraction in the cladding layer 4. When the Al mole fraction increases, the lattice constant of the AlGaN decreases. As a result, greater tensile stress acts on the cladding layer 4, producing cracks in the cladding layer 4.
In addition to the schemes (1) and (2), there is another scheme which improves the optical confinement by increasing the refractive index of the guiding layer 5, not reducing the refractive index of the cladding layer 4. For example, even a slight amount of In can increase the refractive index considerably. The optical guiding layer 5 having a high refractive index can improve the optical confinement without increasing the thickness of the cladding layer 4.
When InGaN is grown by metal organic chemical vapor deposition (MOCVD), pits having a nearly V-shaped cross section occur on the surface. The pits are initiated from the dislocation in the underlying layers and grow in proportion to the thickness of InGaN layer to be grown. To improve the optical confinement for the active layer 6, the optical guiding layer 5 should have a certain degree of thickness. Therefore the very large pits occur in the surface of the optical guiding layer 5. Even if the active layer 6 grown over the optical guiding layer 5 with large bits has a flat surface, the light guided within the waveguide region include the active layer 6, the optical guiding layer 5 and the optical guiding layer 7 is scatterred by the large pits, thus the occurrence of large pits causes the deterioration of laser characteristic. In other words, while the use of InGaN for the optical guiding layer can increase the refractive index of the optical guiding layer, it also increases the scattering loss, thus resulting in an increase in threshold current density.